One-Line Exception Handling

问题:

In Python, it is possible to use one-liners to set values with special conditions (such as defaults or conditions) in a simple, intuitive way.

result = 0 or "Does not exist."  # "Does not exist."

result = "Found user!" if user in user_list else "User not found."

Is it possible to write a similar statement that catches exceptions?

from json import loads

result = loads('{"value": true}') or "Oh no, explosions occurred!"
# {'value': True}

result = loads(None) or "Oh no, explosions occurred!"
# "Oh no, explosions occurred!" is desired, but a TypeError is raised.

回答1:

It is not possible to do a one-line exception-handling statement in python. One could write a function to do this.

def safe_execute(default, exception, function, *args):
    try:
        return function(*args)
    except exception:
        return default

Example usage:

from json import loads
safe_execute("Oh no, explosions occurred!", TypeError, loads, None)
# Returns "Oh no, explosions occurred!"
safe_execute("Huh?", TypeError, int, "10")
#Returns 10

Multiple arguments are supported

from operator import div
safe_execute(
    "Divsion by zero is invalid.",
    ZeroDivisionError,
    div, 1, 0
)
# Returns "Divsion by zero is invalid."

safe_execute(
    "Divsion by zero is invalid.",
    ZeroDivisionError,
    div, 1, 1
)
# Returns 1.

The error-catching process may still be interrupted:

from time import sleep
safe_execute(
    "Panic!",
    Exception,
    sleep, 8
)
# Ctrl-c will raise a KeyboardInterrupt

from sys import exit
safe_execute("Failed to exit!", Exception, exit)
# Exits the Python interpreter

If this behavior is undesired, use BaseException:

from time import sleep
safe_execute("interrupted",
             BaseException,
             sleep, 8)
#Pressing Ctrl-c will return "interrupted"
from sys import exit
safe_execute("Naughty little program!",
             BaseException,
             exit)
#Returns "Naughty little program!"

回答2:

It is possible in one line using exec:

parse_float = lambda x, y=exec("def f(s):\n try:\n  return float(s)\n except:  return None"): f(x)